Synthesis, morphology, and ion conduction of polyphosphazene ammonium iodide ionomers

Anion conducting polyphosphazene ionomer analogues of poly[bis(methoxyethoxyethoxy)phosphazene] (MEEP) were synthesized and their iodide transport properties studied. Polymer bound cations were quaternized with either short alkyl or short ether oxygen chains. X-ray scattering reveals a low q peak near 4 nm^-1 arising from the backbone-backbone spacing between polyphosphazene chains, an ion-related peak at 8 nm^-1, and a peak at 15 nm^-1 corresponding primarily to the amorphous halo of the PEO side chains. Because of the short spacing of the intermediate q peak, the ions are proposed to exist mostly in isolated ion pairs or small aggregates. First-principles calculations combined with dielectric spectroscopy suggest that less than 10% of the ions are in isolated pairs while the remainder participate in quadrupoles or other small aggregates. These ionomers display high values for the high frequency dielectric constant, ε_∞ (highest value ε_∞ = 11), due to atomic polarization of the iodide anion. These MEEP-based ionomers have room temperature dc conductivity of order 10^-6 S cm^-1 and show potential for application in iodide conducting solar cells if the segmental mobility could be increased.